Vcesat (collector-emitter saturation voltage) is an important performance parameter of bipolar transistors, which indicates a required Vce (collector-emitter voltage) value for a transistor with a certain collector current to enter the linear region. Therefore, Vcesat has a direct impact on the DC (direct current) power consumption of a transistor when the transistor operates normally. Reducing Vcesat is a necessary means for a transistor to reduce its DC power consumption, and is especially significant for a high-voltage bipolar transistor. For a radio-frequency high-voltage SiGe HBT, in order to maintain a relatively high cut-off frequency at a high breakdown voltage, it is more practical to increase the distance between pseudo buried layers within a transistor and further to increase the breakdown voltage of the transistor through two-dimensional distribution of a depletion region of collector-base junction, rather than only to increase the thickness of a lightly doped collector region. However, the method causes problems that the increase of the lateral size of the lightly doped collector region accordingly increases the series resistance of the collector region, and thus increases the Vcesat. Especially, the lightly doped collector region under a field oxide region is shallow in silicon, which further increases the series resistance and Vcesat.